FIELD OF THE INVENTION
This invention relates to a foundry process and apparatus using thin rigid closed molds by submerging most or all of the mold below the surface of a molten metal bath, with removal of the mold being handled to hold the metal inside the mold.
High melting point metals and their alloys are traditionally melted in furnaces and then transferred to ladles for gravity pouring into molds of sand, metal, graphite, refractory or ceramic materials. It is sometimes advisable to have molten metal (or alloys) enter the molds directly from the furnace, ladles, or a pouring arrangement, by using vacuum, air or gas pressure, or metal pumps using electromagnetic or other forces to offset gravity (or sometimes to supplement gravity.) Centrifugal force is also used to pressure molten metal into mold cavities as a part of the pouring process.
With some of the aforesaid pouring arrangements, the molten metal or alloy may at some point cover part of a mold's exterior face as with the pouring basin or gate connection. When forming a vacuum seal so that molten metal can be pulled into the mold gates, it is sometimes desirable to immerse the bottom exterior face of the mold in the melt to gain a seal, and even some minor portions of the side exterior faces of the mold can be immersed also for such vacuum seals.
In accordance with the present invention a foundry method comprises assembling and closing an openable rigid mold body represented by metal-confining separable exterior walls defining a mold cavity and substantially uniformly composed of a glass-ceramic converted from the glassy state by devitrification to a polycrystalline ceramic existing substantially as a single phase microstructure, said metal-confining exterior walls having at least one in-gate to admit molten metal into said mold cavity, submerging substantially completely said entire mold body below the surface of a bath of molten metal at a temperature of at least about 2000.degree. F., admitting molten metal from said bath into said mold cavity through said in-gate, disposing said submerged mold in said bath with said in-gate at the highest level of the mold cavity representing the part to be cast, removing said mold body with molten metal therein from said bath while said mold is so disposed, solidifying the metal content of said mold to form a solidified cast metal part, and recovering said solidified cast metal part from said mold.
Also in accordance with the present invention is an apparatus which comprises an openable rigid mold body of metal-confining separable exterior walls defining a mold cavity and substantially uniformly composed of a glass ceramic converted from the glassy state by devitrification to a polycrystalline ceramic existing substantially as a single phase microstructure, said metal-confining exterior walls having at least one in-gate to admit molten metal into said mold cavity, means for closing said openable rigid mold body, means for submerging substantially completely said mold body in a bath of molten meal, means for disposing said mold while in said bath with said in-gate at the highest level of the mold cavity representing the part to be molded, means for removing said mold body from said bath with said in-gate so disposed, means for cooling said removed mold to solidify metal which has entered therein from said bath through said in-gate, and means for removing such solidified metal from said mold.
The process and apparatus of the present invention take advantage of the metallostatic pressure head and the melt's high temperature and radiant heat to preheat the mold, vent its gasses, and to run and feed the casting's sections when the mold is submerged.
The mold itself provides important contributions to the invention. Suitable mold materials which maximize heat transfer in thin wall (up to about one centimeter but preferably on the order of about five millimeters in thickness) mold sections are disclosed in my U.S. Pat. No. 4,411,305. These mold materials provide smooth exterior walls which avoid the problems of metal remaining on the mold exterior after the mold is removed from the melt. Such thin wall molds reduce the mold size required for a given casting and the molten metal displacement necessary for submerged casting. The reduced mold size also simplifies equipment needs since submerged casting requires that all of the mold be below the top surface of the molten metal bath, except for such provision to vent the mold atmosphere and any core gases as may be appropriate. (In this regard it is to be noted that another advantage of the mold sections of my U.S. Pat. No. 4,411,305, is that they are free of any generation of gases per se.
When required, venting can be accomplished by a "chimney" projecting from the mold above the top surface of the bath for as long as it is necessary for venting. The submersion of the mold along with the position and motion given to it uses the heat of the bath and its metallostatic pressure head to preheat the mold and force out the mold atmosphere (and any core or other gases developed) while pressuring the flow of metal into the mold to run even thin, difficult sections. Alternatively, the mold atmosphere may be forced under the metallostatic pressure head into a small cavity or chamber provided at the top of the mold cavity to act as a receiver therefor.
Although the submerged casting process can be carried out by submersion of a mold in a furnace, ladle, trough, or a pouring arrangement containing molten metal, it can also be accomplished by placing the mold in a vessel that is subsequently filled with a molten metal that rises above the mold. The rise of the metal bath around and over the mold can thus be accomplished by lowering the mold or by raising the bath level, or both simultaneously. Both methods utilize the same principle of combined pressure head from the metallostatic source, heat, melt fluidity and its uniformity for venting, gating, running and feeding the submerged mold.
Positioning of the mold vents and mold in-gate(s) will depend on the casting shape and its requirements. Generally vents are most useful when open to the atmosphere during the "gas off." Normally the vent exit is positioned above the in-gate during the initial mold entry into and motion in the melt or bath. "Chimney" type vent extensions can allow the in-gate to be higher than the vent connection to the casting. Once the mold is filled, its removal from the molten bath requires that the in-gate(s) and any feeding reservoirs are above the casting sufficiently to prevent undesired runnout. It is desirable to have the mold leave the melt in a manner that minimizes the length of ingate for removal in subsequent cleaning operations, and in a method that drains any exterior mold pockets that would result in an unwanted "skull" as the metal solidifies.